An 11 inch vacuum forming box, designed to use a standard shop vacuum. This is designed in one piece on a 12" square bed but could be resized for smaller applications, as long as you take the vacuum hose fitting into account. The lid is printed separately. This model took about a roll of PLA to print, so you may want to redo it with thinner sides. I had a full roll of transparent red filament on hand, no particular reason for the color.

If you have woodworking skills and tools, there are ways to make vacuum forming boxes from readily available materials found at big box hardware stores. This plan provides vacuum forming capability for midsize projects without any need for these items.

To form a part, you'll also need a "plug" that is the shape of what you want to mold, and which has a reasonably smooth surface. In this case I'm using a 3d printed plug for an RC plane windshield but it could be anything as long as you can get the plug away from the part you form after the plastic sheet has cooled. There's tons of info on vacuum forming on YouTube so look there for more info on the process in general.